The use of dual containment piping systems in which an inner supply pipe is co-axially placed within an outer secondary containment pipe is known and accepted commercial practice. Typically, such systems are found in the nuclear, oil and gas, petroleum refining and chemical processing industries. The supply pipe is used to transport hazardous or toxic fluid while the secondary containment pipe is used to contain leakage from the supply pipe should it occur. It is also known to provide leakage detectors and drainage systems within the annulus between the carrier and containment pipes.
In the design of petroleum forecourt installations, it is regarded as increasingly important to contain and detect any leaks of petrol or diesel fuel from subterranean pipes which connect one or more storage tanks to dispensing pumps in the installation. To that end, many current designs of forecourt installation utilise secondary containment. This involves containing each fuel supply pipeline in a respective secondary containment pipeline which is optionally sealed at its ends tc the fuel supply pipeline. The secondary containment pipeline prevents leaks from the fuel supply pipeline from being discharged into the environment, and also can convey leaked petrol to a remote-sensing device. Typically, the pipes forming the secondary containment pipeline are initially separate from the fuel pipes and are sleeved over the latter as the fuel pipes are installed between the fuel storage tanks and dispensing pumps.
A wide variety of secondary containment systems are available. Examples are the ENVIROFLEX.TM. piping system available from Total Containment Inc of Exton PA USA and the PERMA-FLEXX.TM. system available from Containment Technologies Corporation Minneapolis USA. A further example is the UPP.TM. pipework system available from PetroTechnik Limited. These systems all share certain common features. Firstly, the primary supply pipe and the secondary containment pipe are of a different construction. Furthermore, they are each available in a range of different diameters to suit different applications. This has the disadvantage that it requires different manufacturing plant to produce each product, primary and secondary, and a substantial amount of stock is required if orders are to be satisfied quickly. Pipe is very bulky to store and thus takes up a great deal of expensive warehousing space.
Such systems therefore require twice as much piping as systems which have no secondary containment, thus increasing the costs of storage and transportation of the components for a forecourt installation. The installation of the secondary containment pipeline increases the amount of time, and hence the cost, of assembling the forecourt installation. In addition, when installing such a system, it can be difficult to ensure that the fuel supply pipes remain spaced from the secondary containment pipes and do not block the passage of leaked fuel to a leak detection system.
Certain types of integral secondarily-contained pipe are known. For example,
GB1326512 (Institut Francais Du Petrole Des Carburants Et Lubricants) describes an inner pipe separated from an outer envelope by a butyl rubber or other type of foam. According to this specification, the foam will expand on contact with any leaked hydrocarbons from the inner pipe so as to seal that pipe. The foam would not therefore appear to be capable of conveying any leaked liquids to a remote sensor. GB1141014 (Samuel Moore & Company) describes a pipe assembly having an inner metal pipe and a heating line running parallel to and along side it, the whole being contained in an outer casing. Fibrous filler material packs the space between the two inner pipes. The fibrous fillers described would absorb any leakage from the primary pipe rather than conveying it to a remote sensor.
U.S. Pat. No. 4,657,050 (Shell Oil Company) illustrates a pipe for conveying hydrocarbons from an offshore location. A steel inner tube runs through a high density plastic sheath formed with circumferential ribs over which a further layer is sleeved. The gaps between the ribs are filled with a foam but the ribs prevent leaked liquid from being conveyed along the pipe. A similar problem exists in the type of pipe disclosed in U.S. Pat. No. 4,644,977 (The Gates Rubber Company) which describes a hose which is formed by co-extrusion and has a foamed layer separating inner and outer non-foamed layers. The foaming process described causes a closed cell foam to form consisting of tiny closed air pockets. Such a foam would positively hamper the progress of any leaked liquid along the pipe.
GB1185062 (Francesco Steffenini) describes a pipe with a one-piece wall having an inner and outer surface, the surfaces being spaced apart by integral helical ribs. The helical arrangement of these ribs provides a positive impediment to the passage of any leaked fluid along the pipe to a remote sensor since any fluid has to make many revolutions of the pipe before reaching an end.